This application note sums up why some power plants and industrial facilities may have issues in exact measurement of fly ash levels. TDR radar sensors from Drexelbrook solves the issues in continuous level measurement. When capacitance and even ultrasonic measurement may fail the DR7100 Radar level transmitter still provides exact measurement.
Application note: Problems with continuous fly ash level measurement
1. Problems with Continuous
Fly Ash Level Measurement
Until the mid 1980’s no one cared too much about the continuous level measurement
and indication of Fly Ash. Fly Ash that was removed from the Precipitators was hauled
off to a “pond” where it was dumped. The pond kept the ash from becoming air borne,
and when one pond was full another was made. This cycle continued for many, many
years.
In the mid 1980’s the EPA decided that just dumping the spent ash in ponds
wasn’t something that would be allowed to continue due to environmental issues.
Coincidently, about this same time, someone discovered that the fly ash had value as
a low cost additive and binder to cement used in road construction. So, the ash was
stored in ash silos and sold to the various companies who had a use, a need and the
willingness to pay for fly ash. So everyone seemed to be happy with the proposed
changes in ash disposal. That is until the silo was full and there was no place to put
the new ash that was constantly being generated by coal fired power plants.
Based on the “new” regulations, if a power plant had no place to put the ash, they
could be forced to shut down, since they could no longer just dump the ash. The
importance of a continuous fly ash measurement was born.
Problems with the continuous level measurement of Ash:
Coal types: Power plants use coal that is mined from many locations both in and
sometimes outside of the US. Some of the different coal types are Anthracite,
Bituminous, and Lignite. The type of coal used depends on the plant’s location and
coal source availability. All plants want to use coal with the highest BTU output, and
the mixing of coal types to get consistent BTU output occurs. Each different coal type
has different physical characteristics in regard to density, dielectric constant, BTU, etc.
The ash that results from this also has different characteristics.
Ash stratification: As different types of coal are burned and the fly ash removed to
storage silos, stratification occurs. The first few feet may be of one type of ash, the
next layer from a different coal/ash type, etc. until the silo is full. Each type of ash layer
has different characteristics of density, dielectric constant, abrasion, etc.
Technology approaches:
• RF / Capacitance - Due to the stratified layers of ash with different dielectric constant this
technology did not produce acceptable results. Level measurements could be off as much as
30% or more. The addition of a series of point level sensors that were tied to the continuous
measurement provided an “Auto-Cal” design that gave somewhat acceptable results but the price
and installation costs were prohibitive.
Power & Co-Gen
Plants Application
420-021-054